Structure-property relationship in waterborne poly(urethane urea)s containing small amounts of graphene derivatives of different nature and surface chemistry

Abstract

Waterborne poly(urethane urea) dispersions are an eco-friendly alternative to solvent-borne adhesives and coatings, but they have low viscosity, reduced tack, and limited mechanical properties. Several additives such as thickening agents, tackifiers, adhesion promoters, or fillers can be used for improving these properties. It has been shown that the addition of different carbon-based fillers (carbon nanotubes, activated carbon, carbon black) improved the thermal, rheological, and mechanical properties of different polyurethane adhesives. However, the addition of these fillers increased the viscosity and stiffness, and reduced the wettability of the polyurethane adhesives, so adding less filler amount without losing performance is desirable. The use of graphene derivative has the potential as a nanofiller for polyurethane adhesives and coatings, because of its high surface area, high aspect ratio, and good mechanical properties. In this doctoral thesis, the influence of adding small amounts (i.e., less than 0.30 wt.%) of graphene derivatives of different nature and surface polarity, during the synthesis of waterborne poly(urethane urea) dispersions (PUDs) on their properties, mainly the adhesion properties, have been considered. This approach is novel in the existing literature and endorsed a better understanding of the structure-property relationship of PUDs containing graphene derivatives. The addition of different small amounts (0.01–0.10 wt.%) of graphene oxide (GO) during the synthesis of PUDs with the acetone method was carried and their properties were characterized. GO was added to polyadipate of 1,4-butanediol polyol and the mixture was incorporated during prepolymer formation in the synthesis of the PUDs. The addition of 0.02–0.04 wt.% GO increased the T-peel strength, whereas the addition of 0.05–0.10 wt.% GO increased the single lap-shear strength. The improved adhesion of the PUDs containing GO was ascribed to the creation of covalent interactions between the surface groups on the GO surface and the NCO groups of the isocyanate during prepolymer formation, creating new urethane groups; during the phase inversion, the covalently bonded GO sheets were embedded between the polyurethane urea chains inside the PUD particles. As a consequence, the adhesion properties of the poly(urethane urea) containing GO films were different in the PU films containing 0.01–0.04 wt.% GO and the ones containing 0.05–0.10 wt.% GO. The addition of the “optimal” amount of GO (0.04 wt.%) in different stages of the synthesis of the PUDs, i.e. before prepolymer formation, after prepolymer formation, and during water addition, were considered. The addition of GO before and after prepolymer formation produced covalent bonds between the GO sheets and the NCO groups of the isocyanate, whereas the GO sheets were trapped between the polyurethane chains when added during the water addition stage. Depending on the stage of the PUD synthesis in which GO was added, the degree of micro-phase separation between the hard and soft segments changed differently, the addition of GO before prepolymer formation changed more efficiently the poly(urethane urea) structure. On the other hand, physical interactions between GO and the poly(urethane urea) chains were produced when GO was added to water during the synthesis, a typical behavior of nanofiller polyurethanes. The role of the surface chemistry of GO on the properties of PUDs containing 0.04 wt.% graphene derivative was assessed. Three different graphene derivatives were selected, graphene oxide – GO -, amine-functionalized GO – A-GO -, and reduced GO – r-GO -, and they were added before prepolymer formation during the synthesis of the PUDs. The addition of the graphene derivatives with different surface chemistry affected differently the structure of the poly(urethane urea)s, because of the different extent of the covalent interactions between the surface groups on the graphene oxide derivatives and the end NCO groups of the prepolymer polyurethane, new urethane groups were formed. The GO and r-GO nano-sheets were covalently bonded to the urethane hard segments. On the other hand, the amine functional groups on the A-GO surface reacted preferentially producing new urea hard domains and because A-GO had thicker stacked graphene nano-sheets, a higher percentage of soft segments than in PU without GO derivative was produced. This led to lower degree of phase separation in the poly(urethane urea). The addition of amine-functionalized graphene oxide and reduced graphene oxide altered the pH value of the waterborne poly(urethane urea)s, the viscoelastic properties of the PU films were also modified by adding 0.04 wt% graphene derivative. Depending on the functional group on the GO derivatives, the extent of shearthinning of PUDs was reduced. However, the PU+GO showed lower thermal stability than other PUs; in addition, the toughness of the PU was improved by adding the GO derivatives. The addition of the GO derivative also affected the mechanical properties of the PUs, as the elongation-at-break increased although the tensile strength and the yield stress decreased. The addition of AGO changed the wettability on the PU film due to the high polarity of the nitrogen functional groups, and higher adhesion properties were obtained in the joints made with PUDs containing GO and r-GO. Finally, small amounts (0.04-0-30 wt.%) of three different graphene derivatives - graphene oxide (GO), graphite nanoplatelets (GP), milled graphite (MG) - having different surface chemistry and number of stacked graphene sheets were added before prepolymer formation during the synthesis of the PUDs. The addition of graphene derivatives changed differently the degree of micro-phase separation of the poly(urethane urea)s depending on the balance between the number of covalent interactions due to the functional groups on the graphene sheets surface and the end isocyanate groups of the prepolymer, and the number of stacked graphene sheets. Whereas MG mainly intercalated between the soft segments showing a typical behavior of nanofiller, GP and, particularly, GO showed different degree of micro-phase separation due to higher number of covalent interactions with the poly(urethane urea) chains. The addition of graphene derivative decreased the glass transition temperature of the hard segments and increased the percentages of associated by hydrogen bond urethane and urea groups, more noticeably by adding GP and MG. Lower temperatures of decomposition and higher amounts of urethane and urea hard domains, and soft domains were found in PU+GO and PU+MG, and they showed an additional structural relaxation at 11-16 ºC. PU and PU+GP had higher thermal stabilities than PU+GO and PU+MG and the addition of GP and, more markedly, GO imparted toughening to the poly(urethane urea). The T-peel strength values were higher in the joints made with PUD+GO, and the lap shear strength of the joints made with PUD+MG and, in less extent, with PUD+GP noticeably increased.